A Christian missionary from the US has entered North Korea carrying a letter to its leader Kim Jong-il in order to call attention to the tens of thousands of political prisoners believed to be held in the communist state.

Robert Park, a 28-year-old Korean-American, was videoed by colleagues walking across the frozen Tumen river into North Korea from China on Christmas Day. He is carrying with him a letter addressed to Kim asking him to release political prisoners and shut down the "concentration camps" where they are held.

It was unclear whether Park had been arrested. Illegal entry into the country is punishable by up to three years in prison. The regime held two American journalists for five months earlier this year, before freeing them during a visit by the former president Bill Clinton.

Park is a missionary from Tucson, Arizona, who works for Pax Koreana, a conservative Seoul-based group that calls for North Korea to improve its human rights record. "I am an American citizen. I brought God's love. God loves you and God bless you," Park was quoted as saying by one of the two activists who filmed him as he crossed the river and who spoke to Associated Press. He said they had last seen Park as he entered North Korea's north-eastern city of Hoeryong from the border late on Friday afternoon. The video footage is expected to be released today.

North Korea holds some 154,000 political prisoners in six large camps across the country, according to South Korean government estimates. The country has long been regarded as having one of the world's worst human rights records, but it rejects outside criticism and denies the existence of prison camps.

"Please open your borders so that we may bring food, provisions, medicine, necessities and assistance to those who are struggling to survive," said the letter, according to a copy posted on the Pax Koreana website. "Please close down all concentration camps and release all political prisoners today."

The activist said that Park also carried a separate written appeal calling for Kim to step down, noting alleged starvation, torture and deaths in North Korean prison camps. That second letter was addressed to the leaders of South Korea, China, the US, Japan and the United Nations. North Korea is expected to react strongly because Park raised the issue of its political system, said Koh Yu-hwan, a professor at Seoul's Dongguk University. Demanding that Kim stepped down was "a kind of hostile act" and "the North won't likely compromise on such an issue", Koh said, predicting it will take time to resolve.

Kim wields absolute power in the communist state of 24 million people. Any acts seen as hostile to him and his leadership carry harsh punishment, said Choi Eun-suk, an expert on North Korean legal affairs at the Institute for Far Eastern Studies at Kyungnam University in Seoul.

The US embassy in Beijing said it was looking into Park's case, but had no details. "His fate to us is unknown," said an embassy spokeswoman, Susan Stevenson. She said a charitable organisation, which she did not identify, had notified the State Department in Washington of Park's actions.

The activist said Park came to South Korea in July and stayed there until leaving for China last week to enter the North. "I would not go to North Korea to live. Even if I die, world leaders should really repent for keeping silent [on North Korea]," Park said in Seoul before leaving for China, the activist said.

The activist said Pax Koreana is affiliated with another organisation called Freedom and Life For All North Koreans, a coalition of advocacy groups for North Korean human rights. Park is a member of the broader group, he said. The coalition and other activist groups plan to hold rallies in New York, Tokyo, Seoul and other cities from today until Thursday.

In August, North Korea released two US journalists sentenced to 12 years' hard labour for trespassing and "hostile acts". Their release came during a trip to Pyongyang by Clinton aimed at winning their freedom. Laura Ling and Euna Lee were captured by guards near the Tumen river in March while reporting a story on North Korean defectors.

Park's reported entry comes weeks after North Korea held one-on-one talks with the US and signalled its willingness to return to international negotiations on ending its nuclear weapons programmes. Pyongyang said this month that it would try to resolve the remaining differences with Washington.